1. Field of the Invention
The present invention relates to a method for improving the oxidation stability of a refined hydrocarbon oil by adding a cyclic sulfide and an aromatic compound to said oil.
2. Description of Related Art
Untreated or virgin hydrocarbon oils are frequently refined to enhance the quality of the oil, e.g. remove sulfur, nitrogen and other contaminants. However, an undesirable consequence of the refining process is that the resulting oil becomes depleted in natural oxidation inhibitors such that the refined oil has poor oxidation stability relative to the untreated oil. Accordingly, one or more compounds are normally added to the refined oil to improve its oxidation stability.
Sulfur compounds are known to improve the oxidation stability of low sulfur refined hydrocarbon oils. For example, U.S. Pat. No. 3,345,381 discloses that cyclic sulfides are useful as oxidation inhibitors in rubber chemistry (see also Table 4 of Bell, C. L. M. et al, "Recent Studies in the Aging of Natural Rubber", Kautschuk und Gummi Kunststoffe, p. 133-137, 19, Jahrgang, Nr. 3, 1966). Acyclic sulfides (i.e. sulfides in which sulfur is not in a ring) are also known as antioxidants (see Rubinshtein, I. A. et al, "Antioxidation Properties of Organosulfur Compounds and a Criterion For Their Evaluation," Khim. i Tekhnol, Topliv i Masel, 45-9 (1965)) as are acyclic sulfides in combination with aromatic hydrocarbons (see U.S. Pat. No. 2,427,766, the disclosure of which is incorporated herein by reference). However, the cyclic and acyclic sulfides normally employed as oxidation inhibitors are quite costly. In addition, the large amounts of sulfides which are required tend to promote the formation of sludge.
Accordingly, it would be desirable to have available a simple and convenient, yet relatively inexpensive, method for reducing the amount of sulfides required while obtaining effective oxidation stability.